The recommended way to do this now seems to be:

  (when display-graphics-p
    (global-unset-key (kbd "C-z")))

Also, though this works with Emacs running as a server, I find it better to just quit emacsclient instead of learning to do something different depending on what environment it is running.

Nope. Again, context matters here. I never use emacs in a terminal; all editting of files on remote systems is done in a local X11 emacs over ssh. And this is not because of circumstance but because making that choice opens up more doors. I can have keybindings not just on C-z but ones that aren't representable in the terminal at all (e.g. I like to put end/beginning-of-buffer on C-. and C-,)

Would I submit my choices as good defaults for new emacs users? Hell no. But they remain good choices nonetheless. That's the essence of my point, not the minutiae.